The invention relates to an apparatus for the determination of the particle size distribution of a granular material by use of a measuring channel equipped with radiation permeable side walls. The granular material is introduced into the measuring channel and, by means of a sifting air current, is fanned out according to the granular size. A scanning apparatus movable outside of the measuring channel is provided consisting of a radiation transmitter and a radiation receiver disposed opposite the transmitter. A measuring beam of this scanning apparatus scans the material blower in a measuring zone lying transversely to the measuring channel.
In a known apparatus of this type (German Pat. No. 2,304,879), the measuring channel consists of a vertically standing, rectangular housing which has flowing therethrough from top to bottom with constant speed a sifting air current into which the granular material to be analyzed is introduced perpendicularly thereto. The local distribution of the particles through the fanning out occurs as the particles move in the measuring channel, occurs according to classification of sizes, and is measured by means of a scanning apparatus. For this purpose, outside of the measuring channel in front of one of its radiation permeable side walls, a radiation transmitter is arranged, and in front of the oppositely disposed side wall is arranged a radiation receiver or detector, respectively, correlated with the radiation transmitter, said radiation receiver or detector receiving the measuring beam. Both the radiation transmitter and the radiation receiver are fixed to a forked carrier enclosing the measuring channel, and the measuring unit consisting of a transmitter, receiver, and common carrier, is drivable by a motor along a guide rail transversely to the direction of flow of the measuring channel through a measuring zone extending at least over the width of the material blower. Accordingly, the measuring beam scans the material blower perpendicularly to a plane of fanning-out.
Through the extinction of the radiation, for example, radiation of light or gamma radiation, the particle concentration or particle size distribution is determined within the measuring zone. For the prevention of measuring errors, the radiation transmitter and radiation receiver must in every position always be exactly flush with one another. The inherent frequency of the scanning apparatus, for the prevention of vibrations, must lie very high, somewhat higher than 5 Hertz. This requirement is achieved with known measuring units in the form of a fork only with difficulty and at great expense. The danger exists that during the scanning of the material blower, falsifications in the measuring values can occur which are caused by vibrations in the scanning apparatus or their measuring units.